1. Field of the Invention
The present invention is directed to magnetic recording media. In particular, the present invention is directed to binder systems for use in such media, to the resulting media obtained by employing such binder systems as well as to methods for increasing the Tg of binder systems.
2. Statement of Art
Magnetic particle recording media generally comprise a non-magnetic substrate onto which a magnetic layer of metal particles or metal oxide particles has been placed. In turn, the magnetic layer is generally prepared from a magnetic particle coating composition comprising a solvent, the metal particles or metal oxide particles, a binder system and other optional ingredients including a lubricant, a dispersant, an abrasive, and the like. Typically, the magnetic particle coating composition is placed on (coated onto) the substrate and the composition cured resulting in the magnetic layer.
Conventional binder systems for use in such magnetic particle recording media include those containing a polyurethane resin, a vinyl chloride/vinyl acetate/vinyl alcohol terpolymer and a polyisocyanate. Upon curing of such binder systems, cross-linking (bonding) between the different binder components can occur, i.e., the hydroxyl groups on both a polyurethane polymer and a vinyl chloride/vinyl acetate/vinyl alcohol terpolymer can react with different isocyanate groups of the same polyisocyanate to form carbamate groups and thereby cross-link with each other through carbamate groups on what was originally the polyisocyanate.
While conventionally employed, these systems nevertheless possess properties which detrimentally affect the resulting magnetic particle recording media. In particular, a desirable characteristic of such media is that the magnetic layer in such media possess a high Tg, i.e., greater than about 30.degree. C. Magnetic layers having a high Tg are particularly beneficial because higher Tg's correlate to improved durability and runnability in the resulting magnetic particle recording media. Additionally, magnetic layers having a higher Tg possess a lower coefficient of friction and are more stiff than magnetic layers with a lower Tg. This latter property is particularly significant as it relates to flexible magnetic particle recording media employed in recording instrumentation wherein a flexible medium, such as a film of polyethylene terephthalate, traverses a recording head because a stiffer film has better head to film contact. Head to film contact is especially critical for thin flexible media.
In spite of the desirability of employing high Tg components in the binder system, use heretofore of such components causes significant problems in the resulting magnetic particle recording media. In particular, when a high Tg polyurethane component is employed in the binder system for use in flexible magnetic particle media, the resulting magnetic layer is not very calenderable. On the other hand, the use of conventional high Tg vinyl chloride/vinyl acetate/vinyl alcohol terpolymers in such binder systems results in other problems. For instance, the use of a conventional low molecular weight, high Tg vinyl chloride/vinyl acetate/vinyl alcohol terpolymer in the binder system results in durability problems in the resulting recording media. Presumably, this problem arises because of the presence of unreacted terpolymer in the binder system which in turn results from the low reactivity of the hydroxyl groups of such terpolymers with the polyisocyanate. In order to overcome this problem arising from the use of such high Tg, low molecular weight terpolymers, it is conventional wisdom that high Tg, high molecular weight vinyl chloride/vinyl acetate/vinyl alcohol terpolymers be employed. Under these circumstances, even if these terpolymers do not react with the polyisocyanate and thereby cross-link with the polyurethane and other terpolymers through the polyisocyanate, durability problems will not arise because the terpolymers are of sufficiently high molecular weight to begin with. However, while use of these high molecular weight, high Tg terpolymers will overcome the durability problems previously encountered, it is also recognized in the art that magnetic layers resulting from coatings which use such terpolymers are more brittle than desired and that the calendering of flexible media employing such coatings generally requires several passes through the rolls in order to achieve an acceptable degree of gloss. However, in spite of these problems, such high molecular weight, high Tg terpolymers are conventionally employed because of the lack of a better alternative. In view of the above, there is a need in the art for a high Tg binder system which overcomes the problems encountered with conventional high Tg binder systems.